1. Field of the Invention
This invention relates generally to electrically connecting one electronic component to another electronic component and, in particular, to providing solder mounds in the form of columns which are connected to the bottom surface metallurgy pads of an electronic component such as a multilayer ceramic package, which component may then be electrically connected to another component.
2. Description of Related Art
Forming an electronic package whereby an electrical component such as a multilayer ceramic package is electrically connected to a substrate such as a card, a board, another connector or any other electronic part is well-known in the art. Surface mount technology (SMT) has gained acceptance as the preferred means of joining electronic devices together, particularly in high end computers. As circuit densities increase there is an increased demand for more I/O and the interconnections which are solder bonded must be smaller and more closely spaced. Additionally, the height of each interconnection must be distributed within a relatively small height tolerance to insure that all of the multilayer ceramic package locations will be electrically interconnected to a corresponding substrate contact location through the solder interconnections.
A myriad of solder structures have been proposed for surface mounting of one electronic structure to another. A typical surface mount process is performed by screening solder paste on conductive, generally metallic pads exposed on a surface of a first electronic structure or substrate. stencil printing operation is used to align a contact mask to the pads. Other known surface mount technology uses solder balls rather than solder paste to provide the solder connecting structures. This technology is known as C-4 and by using solder balls, a more exact and somewhat greater quantity of solder can be applied then through screening. These type structures are used mainly to connect a chip to a multilayer ceramic package.
Another type of solder interconnection is a cast solder column which is mainly used to connect a multilayer ceramic package to a board. Cast columns (CGA) are preferred over pin grid arrays (PGA) in that the nature of the joint (solder connection) offers a better electrical connection over the PGA, is less expensive to manufacture and compensates for thermal expansion differences between the ceramic package and the board. In general, pins are prefabricated and then attached to the bottom side of the package substrate. Sockets are attached to the board where the PGA is plugged in. The PGA process is costly compared to the CGA cast column process.
In the present wire column grid array process a eutectic paste is screened on the substrate pads, typically the bottom surface metallurgy (BSM) pads. The solder columns are loaded separately into a mold or formed in the mold and then aligned to the substrate BSM pads and reflowed. This two-step procedure leads to increased manufacturing cost and possible misalignment of columns to the BSM pads.
One method of forming solder mounds (e.g., columns, etc.) is shown in U.S. Pat. No. 5,244,143, to Ference et al. and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference. The patent discloses an apparatus and method for injection molding solder mounds onto transfer backing plates or electronic devices. In general, the apparatus has a reservoir for molten solder which is disposed over a cavity in an injection plate. The injection plate is disposed over a mold having an array of cavities therein into which solder is injection molded. The mold is typically disposed over a workpiece such as a multilayer ceramic substrate and the cavities in the mold are aligned with electrical contact locations on the substrate. The workpiece is heated and the molten solder is forced under gas pressure into the cavity in the injection plate disposed above the array of cavities in the mold and the molten solder is forced into the cavities. In one embodiment, the injection plate is advanced to slide over the stationary mold to wipe away the excess solder above the mold at a plurality of wiping apertures in the injection plate. The injection plate is then further advanced to a location having a nonsoldered wettable surface at which location the injection plate is removed. The mold is then removed to leave solder mounds disposed on the workpiece.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the present invention to provide a method for molding solder mounds (columns) and to forming solder columns on a substrate for electrically connecting the substrate to another electronic component. The solder columns have layers of one or more different melting point solders.
It is a further object of the present invention to provide an apparatus to fabricate a solder mound (column) in a mold and to fabricate a solder column on an electronic component.
It is another object of the invention to provide a solder mound and in particular a solder column having one or more layers of different melting point solders which mounds and columns may be used to provide interconnection between two electronic components without the need for a solder paste.
Other objects and advantages of the present invention will in part be readily apparent from the following description.